Quaternaryammonium tribromides (QATBs) are highly efficient brominating agents. In other words, these reagents are capable of brominating organic substrates without the use of elemental bromine. QATBs act as vital reagents for preparation of bromoorganics which have anti inflammatory, anti viral (e.g., anti HIV), antineoplastic, antibacterial, and antifungal properties and are capable of acting as flame retardants, agrochemicals and other speciality products. The QATBs are regarded as ‘solid bromine’.
The QATBs are widely used in the bromination of allyl alcohols, enones, alkenes, alkynes, activated aromatics such as anilines and phenols, polycyclic hydrocarbons such as anthracene and phenanthrene and sensitive substrates such as imidazole. They are also used as bifunctional catalysts in the oxidative bromination of aromatic compounds by aqueous hydrogen bromide/hydrogen peroxide, and diastereoselective bromination of allyl glycosides.
While the main claim of the reagents lies in their ability to act as brominating agents, the QATBs are versatile oxidizing agents as well. The reagents are used for selective oxidation of sulfides to sulfoxides, benzyl alcohols to benzaldehydes and benzoic acids, 1,4-benzenediols to 2,5-cyclohexadiene-1,4-diones and oxidation of formic and oxalic acids to carbon dioxide. The oxidation of phosphorus(I) and (III) oxyacids by QATBs leads to the formation of the corresponding phosphorus(III) and (V) oxyacids, respectively.
Highlighting the reagents' diverse uses, the QATBs can also be used efficiently for Hofinann degradation of amides, for methoxybromination of styrene and for the one-step synthesis of N-substituted acylureas and carbamates from amides. Moreover, some of the reagents, CTMATB have anti-bacterial properties as well. In an application of topical importance, these reagents have been shown to work as very effective brominating agents in solvent-free bromination of organic substrates.
Quaternary ammonium tribromides, also referred to as organic ammonium tribromides or organic tribromides or organic ammonium perbromides are well known in the art as witnessed for example by the incorporation of these bromides in the Encyclopedia of Reagents for Organic Synthesis, L. A. Paquette (Ed. in chief), 1995, John Wiley and Sons, Inc., New York and Reagents for Organic Synthesis, L. A. Fieser and M. Fieser, 1967, John Wiley and Sons, Inc., New York.
Reference is made to J. Chem. Soc., 1923, 123, 654 and J. Org. Chem., 1963, 28, 3256, wherein tetramethylammonium tribromide is prepared from tetramethylammonium bromide and bromine in acetic acid. The disadvantages are the toxic effects of bromine and the harsh reaction conditions. Reference is also made to Russian Pat. No. SU 1,113,374 (Cl. CO7C87/30), 15 Sep. 1984, 3,553,280, 28 Dec. 1982, wherein tetraethylammonium tribromide is produced by treating tetraethylammonium monobromide with a brominating agent consisting of a solution of an oxidizing agent such as Ce dioxide hydrate or ceric ammonium nitrate in HBr. The disadvantages are the use of toxic hydrobromic acid and production of heavy metal waste.
Reference is made to J. Am. Chem. Soc., 1951, 73, 4525 wherein preparation of tetrabutylammonium tribromide was achieved by mixing a solution of tetrabutylammonium bromide in carbon tetrachloride with bromine. The disadvantages are the use of highly hazardous bromine and environmentally unacceptable toxic halo solvent. Reference is also made to Bull. Chem. Soc. Jpn., 1987, 60, 2667 wherein benzyltrimethylammonium tribromide is prepared by adding aqueous HBr to a solution of benzyltrimethylammonium chloride and NaBrO3 in water. The disadvantage is the use of environmentally hazardous hydrobromic acid. Reference is further made to Japanese Patent No. 74 14, 411 (Cl. 16 B314); 07 Feb. 1974, Appl. 72 58, 343, 12 Jun. 1972 wherein cetyltrimethylammonium tribromide was prepared by treating the corresponding monobromide with bromine in the presence of aqueous HBr. The disadvantages are the use of toxic chemicals like bromine and hydrobromic acid.